Consider a database with items x and y and assume that there are two transaction
ID: 3776745 • Letter: C
Question
Consider a database with items x and y and assume that there are two transactions T1 and T2 such that: T1 reads items y then x then writes x. Transaction T2 reads items x and y and then writes x and y.
a) In a centralized database environment:
a. Give a schedule with actions of transactions T1 and T2 on items x and y that is not conflict serializable.
b.Give a schedule of T1 and T2 that uses 2PL to ensure serializablity.
c. Give a schedule of T1 and T2 that uses 2PL and results in a deadlock.
b) In a distributed database environment, assume x is stored at site 1 and y is stored at site2:
a. Give serializable local schedules at site 1 and site 2 such that the global schedule is serializable as well.
b. Give serializable local schedules at site 1 and site 2 such that the global schedule is not serializable.
Explanation / Answer
Ans a)
a. schedule for not conflict serializable :
T1:R(Y) T2:R(X) T1:R(X) T2:R(Y) T1:W(X) T2:W(Y) T2:W(X)
b. schedule for 2PL serializablity :
T1
T2
LOCK-S(Y)
LOCK-S(X)
Read(X)
Read(Y)
LOCK-X(X)
LOCK-S(Y)
Read(Y)
Read(X)
Write(X)
LOCK-X(Y)
Write(Y)
Unlock(X)
LOCK-X(X)
Write(X)
Unlock(Y)
Unlock(X)
c.a schedule of T1 and T2 that uses 2PL and results in a deadlock.
T1
T2
LOCK-S(Y)
LOCK-S(X)
Read(X)
Read(Y)
LOCK-S(X)
LOCK-S(Y)
Read(Y)
Read(x)
LOCK-X(X)
T1
T2
LOCK-S(Y)
LOCK-S(X)
Read(X)
Read(Y)
LOCK-X(X)
LOCK-S(Y)
Read(Y)
Read(X)
Write(X)
LOCK-X(Y)
Write(Y)
Unlock(X)
LOCK-X(X)
Write(X)
Unlock(Y)
Unlock(X)
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